MET, or hepatocyte growth factor receptor is a driver and biomarker of numerous cancers. Several kinase inhibitors and extracellular antagonists targeting this signaling axis have demonstrated clinical efficacy in select MET+ patient populations. Moreover, MET overexpression is a predictor of poor prognosis. Thus, the proposed research is centered on the following hypothesis: quantitative molecular imaging of MET expression will provide an efficient means for patient stratification and prognosis by non-invasive assessment of MET levels in primary tumors and metastases. Clinical studies will be needed to appropriately evaluate this hypothesis. The proposed research is designed to generate the molecular imaging probes to enable this assessment while also providing fundamental insight into the characteristics that benefit molecular imaging of solid tumors. Objective: Engineer a sensitive, specific positron emission tomography (PET) agent for MET through protein engineering and comparative evaluation of ligand scaffold topology and biophysical parameters. Aim 1: Comparatively evaluate Gp2, affibody, fibronectin, and antibody synthetic ligands for PET imaging of MET expression in cancer. Synthetic, non-agonistic ligands, with low nanomolar affinities to MET, engineered from three small (5-10 kDa) protein scaffolds will be used as PET imaging agents. 64Cu- radiolabeled ligands will be evaluated in subcutaneous lung cancer xenografts by PET / computed tomography and excised tissue biodistribution. Molecular sensitivity and specificity will be compared across scaffolds and mutants as well as compared to an anti-MET antibody. Metabolite, immunogenicity, and biophysical studies will be performed on lead probes. Aim 2: Enhance the molecular imaging performance of lead probes towards clinical translation through modulation of ligand biophysics, use of a clinically superior radioisotope, and assessment in a more advanced mouse model. Lead imaging agents will be enhanced through the use of a more rapidly decaying radioisotope (68Ga), modified sites and chemistries for radioisotope conjugation, and modulation of ligand hydrophilicity and charge. Molecular sensitivity and specificity will be evaluated in a disseminated lung cancer model in preparation for translation.